By way of a simple explanation the AutoBrake system modulates the amount of hydraulic pressure to the brakes such that the aircraft slows at a predetermined rate of deceleration.
The amount of AutoBrake selected determines the level of that deceleration, and hence the stopping distance, e.g. (and roughly)
AB=1 (flaps 30), brake pressure (nominally 1250 PSI) is modulated to give a deceleration of 4' per sec sqrd and should bring the aircraft to a stop in 8000’ or just over.
AB=2 (flaps 30), brake pressure (nominally 1500 PSI) is modulated to give a deceleration of 5' per sec sqrd and should bring the aircraft to a stop in anything between 6001' to 7999'
AB=3 (flaps 40), brake pressure (nominally 2000 PSI) is modulated to give a deceleration of 7.2' per sec sqrd and should bring the aircraft to a stop in anything between 5001' to 6000'
AB=Max (flaps 40), brake pressure (nominally 3000 PSI) is modulated to give a deceleration of 12' per sec sqrd (below 80 Kts) or 14' per sec sqrd (above 80 Kts) and should bring the aircraft to a stop in 5000' or less.
NB. AutoBrake max does not maintain full brake pressure and as such, whilst it will put the brakes on for you when you touchdown, full toe brake pressure should be applied by the pilot just as soon as possible after touching down - whereby the aircrafts anti-skid system should stop the pilot from locking the wheels ( and accordingly there are big performance penalties associated with having the anti-skid system inoperative ).
Using engine reverse thrust in conjunction with AutoBrake has the effect of reducing / modulating the hydraulic pressure being applied to the brakes, i.e. as the reverse thrust is now helping to slow the aircraft (at the selected AutoBrake decelaration rate) the brakes are now getting some help - and so the autobrake system senses this and releases some of the hydraulic pressure being applied to them.
With normal AutoBrake (1,2,3,Max) you won’t stop any quicker or in any less distance by using reverse thrust – but you will save the brakes.
The other AutoBrake mode is RTO (Rejected TakeOff) - when this is selected (prior to take-off) should the pilots decide to reject a take-off the autobrake system (subject to certain provisos) will apply full unmodulated hydraulic pressure to the brakes and maintain this until the aircraft either comes to a halt and / or the pilots override it - and believe me, it's staggeringly effective - though a big problem after a high speed RTO is the heat in the brakes either melting the fuse plugs and / or starting a fire.
Nb. Of course the stopping distance for any AutoBrake mode is very much dependent upon the condition of the brakes and of the runway.